An internal combustion engine with a torque converter is known that uses a common oil as a hydraulic fluid for the torque converter and a lubricating oil for the internal combustion engine. In the situation where the hydraulic fluid returned from the torque converter in such an internal combustion engine is not discharged into the oil stored in an oil pan, but is directly discharged into a crankcase, the hydraulic fluid returned becomes a mist at a discharge opening, causing a possible reduction in oil circulation efficiency. However, in the situation where an oil passage for the returned hydraulic fluid that is independent of an oil passage for the lubricating oil is formed to discharge the returned hydraulic fluid into the oil stored in the oil pan, the oil passage for the returned hydraulic fluid becomes complicated in structure, causing an increase in number of production steps.
Japanese Patent Laid-Open No. 2003-328717 discloses an internal combustion engine with a torque converter having an oil passage for the hydraulic fluid returned from the torque converter, wherein this oil passage communicates with an oil passage formed along the axis of a crankshaft. A chamber is formed by an oil seal for sealing a shaft bearing provided on the crankshaft. An oil passage is provided that passes through a crankcase and is provided with a check valve at an outlet end. With this configuration, the hydraulic fluid returned from the torque converter can be discharged into the oil stored in the oil pan by a simple change in structure. FIG. 5 illustrates such a prior art configuration, wherein an inlet hole 35a and an outlet hole 36 are formed in a shaft portion 31 of a crankshaft 50. A hydraulic fluid is supplied through the inlet hole 35a into a torque converter 60 and discharged from the torque converter 60 through the outlet hole 36. The crankshaft 50 is provided with a needle shaft bearing 55 for supporting a rotating shaft for the torque converter 60. The inlet hole 35a is located at the axial center of the needle shaft bearing 55. Accordingly, the oil (hydraulic fluid) introduced from the inlet hole 35a is passed through a space between needle rollers and inner and outer rings of the needle shaft bearing 55, and is next supplied to a passage 48.
However, the internal combustion engine with the torque converter disclosed in Japanese Patent Laid-Open No. 2003-328717 has a problem such that when the temperature of the hydraulic fluid for the torque converter is lower than a normal temperature during cold starting or the like and the viscosity of the hydraulic fluid is therefore high, there is a possibility that the hydraulic fluid in the torque converter may not reach a sufficient oil quantity and oil pressure. Thus, a necessary drive force cannot be obtained until the temperature of the hydraulic fluid rises to a certain temperature. This problem is due to the fact that a flow resistance in the vicinity of the inlet hole 35a is increased by an increase in viscosity of the hydraulic fluid, so that the hydraulic fluid is not smoothly supplied into the torque converter.